Hydrogen (H2) is currently the leading candidate for a fuel to replace gasoline/diesel fuel in powering the nation's transportation fleet. There are a number of difficulties and technological barriers associated with hydrogen that must be solved in order to realize this “hydrogen economy”. Inadequate storage systems for on-board transportation hydrogen are recognized as a major technological barrier (see, for example, “The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs,” National Academy of Engineering (NAE), Board on Energy and Environmental Systems, National Academy Press (2004)).
Materials that store hydrogen, and from which hydrogen can be released easily without requiring significant energy input or releasing significant amounts of wasted energy, are of great interest as possible means to enable the hydrogen economy, especially for vehicular transportation. Most materials that chemically store hydrogen, however, either release the hydrogen exothermically (thus require significant energy for their manufacture), or require elevated temperature and the input of significant heat energy for hydrogen release. In either case, the storage loses efficiency.
One of the general schemes for storing hydrogen relates to using a chemical compound or system that undergoes a chemical reaction to evolve hydrogen as a reaction product. In principle, this chemical storage system is attractive, but systems that have been studied to date involve either: (a) hydrolysis of high-energy inorganic compounds where the evolution of hydrogen is very exothermic (sodium borohydride/water as in the Millennium Cell's HYDROGEN ON DEMAND®, and lithium (or magnesium) hydride as in SAFE HYDROGEN®, for example), thus making the cost of preparing the inorganic compound(s) high and life-cycle efficiency low; or (b) dehydrogenation of inorganic hydride materials (such as Na3AlH6/NaAlH4, for example) that release hydrogen when warmed but that typically have inadequate mass storage capacity and inadequate refueling rates.
Inorganic compounds referred to in (a), above, produce hydrogen according to the chemical reactionMHx+XH2O→M(OH)x+XH2  (1)where MHx is a metal hydride, and M(OH)x is a metal hydroxide. This reaction is irreversible.
Inorganic hydride materials referred to in (b), above, produce hydrogen according to the following chemical reaction, which is reversible with H2 (hydrogen gas):MHx=M+x/2H2  (2)where MHx is a metal hydride, M is metal and H2 is hydrogen gas. By contrast to the first reaction, which is irreversible with H2, the second reaction is reversible with H2.
A practical chemical system that evolves hydrogen yet does not suffer the aforementioned inadequacies would be important to the planned transportation sector of the hydrogen economy. This same practical chemical system would also be extremely valuable for non-transportation H2 fuel cell systems, such as those employed in laptop computers and other portable electronic devices, and in small mechanical devices such as lawnmowers where current technology causes significant pollution concerns.
Any heat that must be input to evolve the hydrogen represents an energy loss at the point of use, and any heat that is evolved along with the hydrogen represents an energy loss where the chemical storage medium is regenerated. Either way, energy is lost, which diminishes the life-cycle efficiency. For most organic compounds, such as in those shown in equations 3-5 below, hydrogen evolution reactions are very endothermic, and the compounds are incompetent to evolve hydrogen at ambient temperature (i.e. thermodynamically incapable of evolving H2 at significant pressure at ambient temperature). For temperatures less than about 250-400 degrees Celsius, the equilibrium pressure of hydrogen over most organic compounds is very small. As a consequence, most common organic compounds require heating above about 250 degrees Celsius, and the continual input of high-grade heat to maintain this temperature, in order to evolve hydrogen at a useful pressure.CH4→C+2H2ΔH0=+18 kcal/mol ΔG0=+12 kcal/mol  (3)6CH4→cyclohexane+6H2ΔH0=+69 kcal/mol ΔG0=+78 kcal/mol  (4)cyclohexane→benzene+3H2 ΔH0=+49 kcal/mol ΔG0=+23 kcal/mol  (5)
Most organic compounds have hydrogen evolution reactions that are endergonic (i.e. having a net positive free energy of reaction, i.e. ΔG>0) and their ambient temperature equilibrium hydrogen pressure is very low, practically unobservable. Thus, most organic compounds are unsuitable for hydrogen storage, based on both life-cycle energy efficiency and delivery pressure considerations. Decalin, for example, evolves hydrogen to form naphthalene when heated to about 250 degrees Celsius in the presence of a catalyst (see, for example, “Catalytic Decalin Dehydrogenation/Naphthalene Hydrogenation Pair as a Hydrogen Source for Fuel-Cell Vehicle,” Hodoshima et al., J. Hydrogen Energy (2003) vol. 28, pp. 1255-1262, incorporated by reference herein). Hodoshima et al. use a superheated “thin film” reactor that operates at a temperature of at least 280 degrees Celsius to produce hydrogen from decalin at an adequate rate. Thus, this endothermic hydrogen evolution reaction requires both a complex apparatus and high-grade heat, which diminishes the life-cycle energy efficiency for hydrogen storage.
Boranes (i.e. borane compounds) have high hydrogen storage capacities and have attracted interest for use as hydrogen storage materials for transportation, but the difficulty of manufacturing borane compounds, and the life-cycle energy inefficiency of their present manufacture chemical processes, prevents their widespread use.
Methods and systems that employ chemical compounds for storing and evolving hydrogen at ambient temperature with minimal heat input remain highly desirable.